Você precisa criar macros que mostrem e, em seguida, oculte as planilhas:
Sub ShowSheet()
WorkSheets("SheetName").Visible = xlSheetVisible
End Sub
Sub HideSheet()
WorkSheets("SheetName").Visible = xlSheetHidden
End Sub
Seu sub principal será:
Sub Auto_Run()
Application.OnTime TimeValue("15:00"), "ShowSheet"
Application.OnTime TimeValue("15:30"), "HideSheet"
End Sub